Semiconductor devices produced by forming an impurity region of an intended conductivity type by introducing an impurity into the surface of a semiconductor substrate such as a silicon substrate have been miniaturized, and it is accordingly needed to precisely form shallow impurity regions in minute areas. Known techniques for introducing an impurity into the surface of a solid material include a plasma doping (PD) method of introducing an impurity into the solid material with a low energy after ionizing the impurity (see, for example, Patent Document 1).
Moreover, in order to enhance the reproducibility of the dose to be introduced into a semiconductor substrate in a plasma doping process, the present inventors have proposed a plasma doping method in which discharge is repeated over a certain period of time after the maintenance of the plasma doping apparatus to provide a preliminary period for stabilizing the dose with respect to the accumulated discharge time (see, Patent Document 2).
Specifically, Patent Document 2 discloses that in a plasma doping process using a B2H6/He plasma, for example, a boron-containing film is gradually formed inside the vacuum chamber in a continuous discharge operation, whereby the boron dose with respect to the accumulated discharge time of plasma doping increases, and that the increase in the boron dose with respect to the accumulated discharge time gradually decreases as the continuous discharge operation is continued, wherein there exists a period of time over which the boron dose is substantially constant with respect to the increase in the accumulated discharge time. Patent Document 2 also discloses that it is possible to obtain a stable dose by using, as the process window, a period of time over which the dose is substantially constant, independent of the accumulated discharge time. FIG. 24 is a flow chart of a plasma doping method disclosed in Patent Document 2 in which the dose can be controlled. As shown in FIG. 24, the plasma doping apparatus is maintained in step S1001, and then the dose to be introduced into the subject substrate is determined by using, as the process window, a period of time over which the dose is substantially constant, independent of the accumulated discharge time in step S1002. Then, in step S1003, a plasma doping process is performed using the dose determined in step S1002 as a target value. Then, after removing a resist used as a mask when introducing an impurity, an annealing process is performed for activating the impurity introduced in the subject substrate.
Patent Document 1: U.S. Pat. No. 4,912,065
Patent Document 2: International Publication WO06/121131